This invention relates to an image forming apparatus capable of selectively copying an image of a document (document image) on a plurality of types of copy items such as normal paper sheets, tracing paper, film sheets, and the like.
Image forming apparatuses such as copying machines have been conventionally such that a document image is copied on a copy sheet at a copying magnification set by operating an operating unit.
These apparatuses normally operate as follows. The document image is read optically by an optical assembly including an optical system to form an electrostatic latent image on the circumferential surface of a photosensitive drum. Charged toner is supplied to the latent image, thereby developing the latent image into a toner image. This toner image is transferred to a copy sheet, which is heated while being pressed against the copy sheet in a fixing device so that the toner image is fixed thereon.
Having the property of shrinking upon being subjected to the heat, the copy sheet is subject to the thermal shrinkage when passing through the fixing device, which results in a change in the size of the copy sheets. Since the document image formed on the copy sheet shrinks as the copy sheet shrinks, there is a discrepancy between the set nominal copying magnification and an actual magnification of the document image formed on the copy sheet. Particularly in the copying operation where the magnification is life-size or 1, it can be seen easily that the size of the image on the document and that on the copy differ, resulting in the reduced reliability.
Thus, in the conventional image forming apparatuses, the optical system is adjusted so that the magnification of the document image fixed to a normal paper sheet which is frequently used as a copy sheet corresponds with the set nominal copying magnification.
However, instead of normal paper sheets, tracing paper and film sheets are sometimes used for the copying operation and the degree of thermal shrinkage differs depending upon the type of copy sheet. For example, upon being subjected to the heat, a 2000 mm long normal paper sheet shrinks 2 mm and a 2000 mm long film sheet shrinks as much as 10 mm as shown in TABLE-1 to be described later. Thus, with the image forming apparatus wherein the optical assembly is adjusted based on the normal paper sheet, there is a large discrepancy between the magnification of the document image on the film sheet and the set nominal copying magnification.
In order to avoid this problem, an operator has been forced to adjust the optical system through trial and error when a document image is copied on a film sheet. This is disadvantageous because a cumbersome adjustment is necessary each time the film sheet is used and the copy sheet is wasted upon the improper adjustment.